Implantation of stents has become established as one of the most effective therapeutic measures for treatment of vascular diseases. Stents assume a supporting function in the hollow organs of a patient. Stents of conventional construction have a base body with a plurality of circumferential support structures. For example, metallic struts have a base body which is initially in a compressed form for insertion into the body and then is dilated at the site of use. One of the main areas for use of such stents is for permanently or temporarily widening and keeping open of vascular obstructions, in particular constrictions (stenoses) of the coronary vessels. In addition, aneurysm stents are known that serve to support damaged vascular walls or seal off intracerebral vascular bulges.
Conventional stents for the treatment of stenoses have a cylindrical base body of sufficient load-bearing capacity that opens the constricted vessel and keeps it open to the desired degree to restore unobstructed blood flow. The circumferential wall of the base body is typically formed by a lattice-like bearing structure, allowing for the stent to be inserted in a compressed (crimped) state with a small outside diameter up to the point of constriction of the vessel to be treated, and to be sufficiently widened, e.g., by means of a dilatation balloon catheter until the vessel has the desired increased inside diameter. The steps of placing and expanding the stents during this procedure and their final positioning in the tissue upon completion of the procedure must be monitored by the cardiologist. This may be accomplished by means of imaging methods such as x-ray examinations.
The stent has a basic body made of an implant material. An implant material is a nonviable material that is used in medicine and interacts with biological systems. The basic prerequisite for the use of a material as implant material that is in contact with the physical body environment during its intended use is its physical compatibility (biocompatibility). Biocompatibility refers to the ability of a material to induce an appropriate tissue reaction in a specific application. This includes adaptation of the chemical, physical, biological and morphological surface properties of an implant to the recipient tissue with the goal of clinically desirable interaction. The biocompatibility of the implant material further depends on the chronological course of reaction of the biosystem in which it is implanted. Irritations and inflammations may occur at relatively short notice and cause tissue changes. Biological systems thus react in different ways, depending on the properties of the implant material. According to the reaction of the biosystem, implant materials may be categorized as bioactive, bioinert and biodegradable/resorbable materials.
Stents have a cylindrical base body including a lumen along the axial direction. The base body has a plurality of meander-shaped struts, forming the circumferential support structures, e.g. circumferential cylindrical meandering rings or helices, arranged one after the other along the axial direction. The support structures are connected in the axial direction by means of connecting elements, so-called axial connectors or connectors. At least in vascular support stents these axial connectors must on the one hand be arranged in such a manner that sufficient bending flexibility of the stent is guaranteed, and on the other hand they should not obstruct the crimping and/or dilatation processes.
U.S. Pat. No. 6,464,720 proposes a stent design in which the stent base body has apertures. These apertures serve to accommodate radiopaque markers made of a material that does not allow the passage of x-rays. While the apertures in this stent design only minimally affect crimpability, they hinder homogenous plastic deformation of the support elements and thus have a significant negative impact on the mechanical properties of the stent.
A cause for increased vascular inflammatory reactions upon stent implantation is the targeted use of stent overdilatation, which is necessitated by a certain spring-back of the stent shortly after implantation, so-called recoil. Such recoil, whose degree depends on the respective design and, particularly, the material used, is shown by any material composition used for implants. To achieve a minimum lumen size that is physiologically reasonable for the treated vessel after implantation, overdilatation of the stent is necessary to offset recoil. This overdilatation causes the vessel to be overstretched so that vessel damage occurs, causing the body to respond with an inflammatory reaction and subsequent increased formation of new tissue (neointimal proliferation). Both reactions need to be minimized in the context of stent implantations.
Especially when using magnesium or a magnesium alloy as a degradable stent material, it is particularly important, due to their not very favorable mechanical material properties, to minimize the effects on the distribution of forces, combined with an effective utilization of crimp space, which calls for optimal design of the axial connectors.